A
quantum
computer
could,
in
principle,
outperform
a
classical
computer
by
exploiting
the
ability
of
a
quantum
system
to
be
in
two
states
–
often
called
0
and
1
–
at
the
same
time.
When
two
qubits
are
entangled,
they
behave
as
one
system:
this

means
that
the
quantum
state
of
one
qubit
directly
depends

on
the
state
of
the
other.
It
was
once
thought
that

entanglement
was
only
possible
with
individual
quantum

particles
–
such
as
photons
–
but
recent
experiments
have

shown
that
macroscopic
objects
can
also
be
entangled.

Superconducting
materials
are
good
for
making
qubits

because
decoherence
effects
–
which
wipe
out
quantum

behaviour
–
can
be
limited.

Berkley
and
co-workers
made
their
qubits
from
a
Josephson

Junction
–
a
type
of
superconducting
”reservoir”
–
and
coupled
two
qubits
together
using
a
capacitor.
Under
certain

conditions,
the
qubits
can
exist
in
one
of
two
states:
a
ground

state
or
an
excited
state.
When
the
two
qubits
are
entangled,

if
qubit
1
is
in
the
ground
state
then
qubit
2
is
in
the
excited

state,
and
vice
versa.

The
researchers
measured
the
entangled
states
by
applying

microwaves
to
the
system
and
recording
transitions
from
the

ground
state
to
higher
energy
states.
“Such
evidence
for

entanglement
over
a
macroscopic
length
is
particularly

promising
for
the
construction
of
a
quantum
computer,
as
this
will
require
many
spatially
separated
qubits,”
said
Berkley.